1. Field of the Invention
The present invention relates generally to bandpass filters for cellular radio transmitters. More particularly, it relates to bandpass filtering for reducing phase noise and spurious noise.
2. Description of Related Art
The Global System for Mobile Communications (GSM), originally known as Groupe SpÈciale Mobile, was developed as a standard for cellular communication in Europe. GSM offers a wide range of functionality in the transmission of voice and data. It has been adopted throughout Europe and the rest of the world. The original primary GSM has been expanded to encompass other versions for wider total bandwidths, such as extended GSM, DCS 1800 and PCS 1900.
GSM is a purely digital system. The primary GSM system uses the 900 MHz band, of which 890-915 MHz is for mobile transmissions, and 935-960 MHz is for the base transmissions. There are 124 channels (174 channels for extended GSM, 374 channels for DCS 1800 or PCS 1900), with each channel being 200 kHz wide. In GSM, TDMA (time division multiplexing access) is used, with 8 time slots per channel, numbered 0 to 7. GSM uses wide channels to allow high speed digital transmission, resulting in reduced effects of fading and minimizing production costs. Gaussian Minimum Shift Keying (GMSK) is used as the modulation process.
In a typical communication system using GSM, there are a number of processing stages involving modulation and filtering. Modulation converts intermediate frequency (IF) signals to radio frequency (RF) signals. Filtering, including bandpass filtering, eliminates the unwanted frequency components such as various forms of noise. In general, at a cellular radio transmitter, an information bit sequence is coded and transformed into an analog waveform centered at baseband frequency, which is subsequently up-converted to be centered at a higher frequency in order to be transmitted over one of a number of predetermined radio frequency (RF) channels.
Up-conversion techniques usually produce a signal with higher phase noise and spurious noise than is acceptable. Thus, bandpass filtering is needed to suppress these noise components before the signal is power-amplified for transmission. In a typical prior art system, a bandpass filter, usually a conventional surface acoustic wave (SAW) bandpass filter, is used to filter out the phase noise and spurious noise in the RF signal. Due to the fixed, wide bandwidth of the filter, if the center frequency of a signal is near one of the two edges of the filter""s pass band, the bandpass filter will not provide the same attenuation in the two sidebands of the signal. As a result, the attenuation will not be symmetrical about the center frequency of the bandpass filter. This unequal attenuation causes unwanted sidebands in the signal spectrum and in turn causes the carrier envelope of the filtered output signal to be non-constant. It is well known that, if the carrier envelope is non-constant, there will be sidelobe regrowth after the signal is amplified for transmission by the power amplifier. This is due to the non-linearities of the power amplifier. In order to reduce these regrown sidelobes, more filtering is needed after the power amplification. The additional filtering is typically performed by additional components such as a bandpass duplexer. This bandpass duplexer results in further power loss and increases the system cost.
Accordingly, there is a need for a bandpass filter which: (1) is more effective in a cellular transmit path than a conventional bandpass filter, (2) does not affect the approximately constant carrier envelope of the input signal, and (3) is cost effective.
The present invention comprises methods and apparatus for bandpass filtering a radio frequency (RF) signal in a cellular transmit path to provide a filtered RF signal to a power amplifier. The methods comprise bandpass filtering the RF signal by tracking, without translating, the carrier frequency of the RF signal with a phase-locked loop prior to power amplifying the signal for transmission. The tracking filter provides symmetrical filtering by centering the pass band of the tracking filter on the RF carrier, irrespective of where the RF carrier is within the pass band of the system.